In the ACP, go to "General > Categories and forums", and select the forum you wish to apply the rules to. Scroll down to the bottom, and you will see "Forum Rules".Just beneath that, you will see "Show rules on". Choose your selection (Choices: Topics creation, Standard replies, Quick reply), and In the "Message" field below, enter your rules and hit save.

To make It more noticeable, you can also add an Image In the "Rules Image URL" field just below the Message field.